# BlueGreen Deployment Strategy

A Blue Green Deployment allows users to reduce the amount of time multiple versions are running at the same time.

## Overview

In addition to managing ReplicaSets, the rollout controller will modify a Service resource during the `BlueGreenUpdate` strategy. The Rollout spec has users specify a reference to active service and optionally a preview service in the same namespace. The active Service is used to send regular application traffic to the old version, while the preview Service is used to funnel traffic to the new version. The rollout controller ensures proper traffic routing by injecting a unique hash of the ReplicaSet to these services' selectors.  This allows the rollout to define an active and preview stack and a process to migrate replica sets from the preview to the active. 

When there is a change to the `.spec.template` field of a rollout, the controller will create the new ReplicaSet.  If the active service is not sending traffic to a ReplicaSet, the controller will immediately start sending traffic to the ReplicaSet. Otherwise, the active service will point at the old ReplicaSet while the ReplicaSet becomes available. Once the new ReplicaSet becomes available, the controller will modify the active service to point at the new ReplicaSet. After waiting some time configured by the `.spec.strategy.blueGreen.scaleDownDelaySeconds`, the controller will scale down the old ReplicaSet.

!!! important
    When the rollout changes the selector on a service, there is a propagation delay before all the nodes update their IP tables to send traffic to the new pods instead of the old. During this delay, traffic will be directed to the old pods if the nodes have not been updated yet. In order to prevent the packets from being sent to a node that killed the old pod, the rollout uses the scaleDownDelaySeconds field to give nodes enough time to broadcast the IP table changes.

!!! important
    ALB Ingress with Rollouts blue-green strategy is not supported without a chance of downtime.

    When using an AWS ALB to route traffic to a service, the ALB Ingress Controller does not update the target groups in an atomic or safe manner. This can result in a situation where, during a deployment, the stable target group temporarily has no pods registered. This occurs because the ALB Controller removes all current pods from the target group before registering pods from the desired ReplicaSet.
    The desired pods must pass their initial configured health check on the stable target group to be considered healthy by the ALB. This creates a risk where the ALB may temporarily have no healthy pods registered to the target group, depending on the timing of deregistration and registration of new pods. This can lead to application downtime that the rollouts controller cannot prevent.

## Example

```yaml
apiVersion: argoproj.io/v1alpha1
kind: Rollout
metadata:
  name: rollout-bluegreen
spec:
  replicas: 2
  revisionHistoryLimit: 2
  selector:
    matchLabels:
      app: rollout-bluegreen
  template:
    metadata:
      labels:
        app: rollout-bluegreen
    spec:
      containers:
      - name: rollouts-demo
        image: argoproj/rollouts-demo:blue
        imagePullPolicy: Always
        ports:
        - containerPort: 8080
  strategy:
    blueGreen: 
      # activeService specifies the service to update with the new template hash at time of promotion.
      # This field is mandatory for the blueGreen update strategy.
      activeService: rollout-bluegreen-active
      # previewService specifies the service to update with the new template hash before promotion.
      # This allows the preview stack to be reachable without serving production traffic.
      # This field is optional.
      previewService: rollout-bluegreen-preview
      # autoPromotionEnabled disables automated promotion of the new stack by pausing the rollout
      # immediately before the promotion. If omitted, the default behavior is to promote the new
      # stack as soon as the ReplicaSet are completely ready/available.
      # Rollouts can be resumed using: `kubectl argo rollouts promote ROLLOUT`
      autoPromotionEnabled: false
```

## Configurable Features
Here are the optional fields that will change the behavior of BlueGreen deployment:
```yaml
spec:
  strategy:
    blueGreen:
      autoPromotionEnabled: boolean
      autoPromotionSeconds: *int32
      antiAffinity: object
      previewService: string
      prePromotionAnalysis: object
      postPromotionAnalysis: object
      previewReplicaCount: *int32
      scaleDownDelaySeconds: *int32
      scaleDownDelayRevisionLimit: *int32
```

## Sequence of Events

The following describes the sequence of events that happen during a blue-green update.

1. Beginning at a fully promoted, steady-state, a revision 1 ReplicaSet is pointed to by both the `activeService` and `previewService`.
1. A user initiates an update by modifying the pod template (`spec.template.spec`).
1. The revision 2 ReplicaSet is created with size 0.
1. The `previewService` is modified to point to the revision 2 ReplicaSet. The `activeService` remains pointing to revision 1.
1. The revision 2 ReplicaSet is scaled to either `spec.replicas` or `previewReplicaCount` if set.
1. Once revision 2 ReplicaSet Pods are fully available, `prePromotionAnalysis` begins.
1. Upon success of `prePromotionAnalysis`, the blue/green pauses if `autoPromotionEnabled` is false, or `autoPromotionSeconds` is non-zero.
1. The rollout is resumed either manually by a user, or automatically by surpassing `autoPromotionSeconds`.
1. The revision 2 ReplicaSet is scaled to the `spec.replicas`, if the `previewReplicaCount` feature was used.
1. The rollout "promotes" the revision 2 ReplicaSet by updating the `activeService` to point to it. At this point, there are no services pointing to revision 1
1. `postPromotionAnalysis` analysis begins
1. Once `postPromotionAnalysis` completes successfully, the update is successful and the revision 2 ReplicaSet is marked as stable. The rollout is considered fully-promoted.
1. After waiting `scaleDownDelaySeconds` (default 30 seconds), the revision 1 ReplicaSet is scaled down 


### autoPromotionEnabled
The AutoPromotionEnabled will make the rollout automatically promote the new ReplicaSet to the active service once the new ReplicaSet is healthy. This field is defaulted to true if it is not specified.

Defaults to true

### autoPromotionSeconds
Setting a positive non-zero value here would make the rollout automatically promote the new `ReplicaSet` to active Service after this much time has been elapsed since the rollout has entered a paused state. If the `AutoPromotionEnabled` field is set to **false**, this field would be ignored.

Defaults to nil

### antiAffinity
Check out the [Anti Affinity document](../anti-affinity/anti-affinity/) for more information.

Defaults to nil

### maxUnavailable
The maximum number of pods that can be unavailable during the update. Value can be an absolute number (ex: 5) or a percentage of desired pods (ex: 10%). This can not be 0 if MaxSurge is 0.

Defaults to 0

### prePromotionAnalysis
Configures the [Analysis](analysis.md#bluegreen-pre-promotion-analysis) before it switches traffic to the new version. The
AnalysisRun can be used to block the Service selector switch until the AnalysisRun finishes successful. The success or
failure of the analysis run decides if the Rollout will switch traffic, or abort the Rollout completely.

Defaults to nil

### postPromotionAnalysis
Configures the [Analysis](analysis.md#bluegreen-pre-promotion-analysis) after the traffic switch to the new version. If the analysis
run fails or errors out, the Rollout enters an aborted state and switches traffic back to the previous stable Replicaset.
If `scaleDownDelaySeconds` is specified, the controller will cancel any AnalysisRuns at time of `scaleDownDelay` to 
scale down the ReplicaSet. If it is omitted, and post analysis is specified, it will scale down the ReplicaSet only 
after the AnalysisRun completes (with a minimum of 30 seconds).

Defaults to nil

### previewService
The PreviewService field references a Service that will be modified to send traffic to the new ReplicaSet before the new one is promoted to receiving traffic from the active service. Once the new ReplicaSet starts receiving traffic from the active service, the preview service will also be modified to send traffic to the new ReplicaSet as well. The Rollout always makes sure that the preview service is sending traffic to the newest ReplicaSet.  As a result, if a new version is introduced before the old version is promoted to the active service, the controller will immediately switch over to that brand new version.

This feature is used to provide an endpoint that can be used to test a new version of an application.

Defaults to an empty string

### previewReplicaCount
The PreviewReplicaCount field will indicate the number of replicas that the new version of an application should run.  Once the application is ready to promote to the active service, the controller will scale the new ReplicaSet to the value of the `spec.replicas`. The rollout will not switch over the active service to the new ReplicaSet until it matches the `spec.replicas` count.

This feature is mainly used to save resources during the testing phase. If the application does not need a fully scaled up application for the tests, this feature can help save some resources.

If omitted, the preview ReplicaSet stack will be scaled to 100% of the replicas.

### scaleDownDelaySeconds
The ScaleDownDelaySeconds is used to delay scaling down the old ReplicaSet after the active Service is switched to the new ReplicaSet.

Defaults to 30

### scaleDownDelayRevisionLimit
The ScaleDownDelayRevisionLimit limits the number of old active ReplicaSets to keep scaled up while they wait for the scaleDownDelay to pass after being removed from the active service. 

If omitted, all ReplicaSets will be retained for the specified scaleDownDelay